This invention relates to alignment and isolation apparatus and methods for use particularly in microlithography, among other applications. More particularly, this invention is directed to an apparatus which efficiently, electromagnetically supports a stage for aligning a stage in a microlithography machine.
The need for precise positioning of an object is required in many fields of application, including applications in semiconductor manufacturing such as microlithography. As microprocessors become faster and more powerful, an ever increasing number of transistors are required to be positioned on a semiconductor chip. This necessitates closer placement of the transistors and circuits interconnecting them, which in turn requires an ever increasing accuracy in the methods for laying down the circuits on the chip. Thus, there is a need for more precise positioning and maintaining of position, of a substrate during microlithography.
Various systems have been designed to attempt to improve fine positioning and movement control of a workpiece. British Patent Specification 1,424,413, assigned to Handotai Kenkyo Shinkokai describes several stages that are supported by flexures and actuated using electromagnetic force actuators. U.S. Pat. No. 3,935,486, invented by Nagashima describes a stage that is controlled using electromagnetic force actuators. In this case, the stage is supported on flexural bearings in 6 degrees of freedom (DOF) and the actuating portions are used to adjust the position of the stage. Both of these designs utilize flexural bearings to constrain the motion of the stages in 6 DOF. The electromagnetic actuator devices only provide force; they are not used to control all directions of motion of the stage.
Ideally, the bearings for a stage should have infinite stiffness in the directions for which position of the stage is to remain fixed, and zero stiffness along the directions in which the stage is to be moved, to maximize precision and efficiency. Flexural bearings fall far short of the ideal and generally have a stiffness ratio (stiffness in directions to be fixed to stiffness in directions to be moved) of only about 100:1 and possibly up to about 1000:1 but the price of the latter is likely prohibitive in practice. Moreover, a much greater stiffness ratio is desirable.
U.S. Pat. No. 4,952,858 invented by Galburt describes a wafer fine stage that is supported and positioned in 6 DOF by electromagnetic voice coil motors. The motion of the wafer fine stage is entirely constrained using voice coil motors, and this design does not utilize any flexural bearings. Voice coil motors, however, require relatively large amounts of power to generate a given amount of force. The high power requirements of voice coil motors can generate sufficient heat to change the index of refraction of the environment sufficiently to induce error in an interferometer system. Additionally, heat generation can cause expansion of the stage leading to further errors in alignment and control. Further, U.S. Pat. No. 4,952,858 discloses the use of permanent magnets to counterbalance the weight of the fine stage. This counterbalance force is a nonlinear function of stage position, and is thus quite difficult to control accurately.
U.S. Pat. Nos. 5,157,296 and 5,294,854, invented by Trumper describe a wafer fine stage bearing system. This system includes electromagnetic actuator devices, which act as bearings in 6 DOF. These patents describe control means for the bearings and apparatus for counterbalancing the weight of the stage using either opposed permanent magnets or a heavy oil in which the stage floats. U.S. Pat. Nos. 5,157,296 and 5,294,854 also do not utilize flexural bearings. The electromagnetic actuator devices in the Trumper patents are arranged in pairs, on opposite sides of the stage, in order to provide stable control. Thus, all forces applied by the electromagnetic pairs are transmitted through the stage, which can result in deformation of the stage.
The counterbalance forces in the Trumper patents may be provided by permanent magnets or by floating the stage in oil. As noted above with regard to the Galburt patent, utilization of permanent magnets results in a nonlinear force curve and corresponding control problems. With regard to floating the stage in oil, oil presents significant problems for a clean room environment typically used for semiconductor processing.
U.S. Pat. No. 5,528,118, invented by Lee, describes a guideless stage for aligning a wafer in a microlithography system, and a reaction frame which isolates both external vibrations as well as vibrations caused by reaction forces from an object stage.
U.S. Pat. No. 5,623,853, invented by Novak, et al., describes a wafer coarse and fine stage for a lithography machine. The coarse stage is a stacked arrangement of linear motor-driven air bearing slides. The fine stage is driven in 3 DOF using voice coil motors. The remaining DOF of the fine stage are constrained using flexural bearings. The use of flexural bearings for the 3 planar DOF limits the servo bandwidth of the stage because the flexural bearings have a limited stiffness in the plane. In addition, the finite stiffness of the flexural bearings out of the plane, distorts the out of plane motion of the stage.
In accordance with principles of the invention a stage is provided which has a main surface positionally controllable in at least one degree of freedom. At least one pair of electromagnetic actuator devices couples the stage to a supporting stage for controlling movement of the stage in at least one degree of freedom. Both actuating portions of each pair of electromagnetic actuator devices are mounted adjacent to a single side of the stage so as not to distort the stage during controlling movements thereof.
Both of the actuating portions of each pair are mounted on the supporting stage in close opposition to one another, and a pair of corresponding targets are mounted on the stage adjacent one another and within a predefined gap defined by the pair of mounted electromagnetic actuator devices. Preferably, the stage is magnetically coupled, born and positionally controlled in three degrees of freedom by three pairs of electromagnetic actuator devices which electromagnetically couple the stage to the support stage. Two of the pairs are preferably aligned substantially parallel to a first direction, and the third pair is aligned in a second direction substantially perpendicular to the first direction. The first and second directions are substantially within the plane of the main surface of the stage.
A pair of corresponding targets are peripherally mounted on the stage for interaction with each electromagnetic pair mounted on the supporting stage. The pairs of electromagnetic actuator devices preferably comprise variable reluctance actuating portions which provide a very favorable force to surface area capability compared to other types of electromagnetic actuator devices, resulting in less heat generation. The three pairs of electromagnetic actuator devices interconnecting the stage and the supporting stage are actuable to control the stage in three degrees of freedom.
Three additional electromagnetic actuator devices are mounted between the stage and the supporting stage, which are actuable to control the stage in three additional degrees of freedom. The additional electromagnetic actuator devices preferably comprise voice coil motors, since the requirements for control in the three additional degrees of freedom are less stringent than those required of the electromagnetic pairs. Each of the three additional electromagnetic actuator devices preferably comprises one pair of electromagnetic actuator devices each comprising an actuating portion of variable reluctance and a corresponding target portion, when heat generation from the electromagnetic actuator device is critical to the system in which the stage is employed. The additional electromagnetic actuator device may be accompanied by a supplemental vertical support disposed at the same location as the additional electromagnetic actuator device so as not to cause any undesirable distortion to the stage. The supplemental vertical supports preferably comprise air bellows but may also be air cylinders or the like.
In use, the stage is levitated above the support stage by at least one non-contact vertical support member which is preferably three non-contact vertical support members for controlling the position of said stage in three vertical degrees of freedom. The non-contact vertical support members are preferably electromagnetic actuator devices, most preferably voice coil motors.
The targets are preferably mounted peripherally of the main surface of the stage, on target mounts which may be U-shaped, or the like, webs of material extending from the stage. Pairs of targets are mounted on each of the target mounts such that only a resultant force from actuation of the pair of corresponding electromagnetic actuator devices is transferred to the stage through each target mount.
Further disclosed is a lithography system incorporating the high accuracy stage according to principles of the invention. The lithography system includes an illumination system mounted on a frame, a supporting stage mounted on the frame which is substantially aligned with the illumination system, a stage having a main surface positionally controllable in at least one degree of freedom, and at least one pair of electromagnetic actuator devices coupling the stage to the supporting stage for control in at least one of the degrees of freedom. Both actuating portions of the pair of electromagnetic actuator devices are mounted adjacent a single side of the stage.
The lithography system farther includes a mask pattern positioned between the illumination system and the stage, a reticle supporting the mask, and optical system positioned between the reticle and the stage. The stage is preferably positionally controllable in at least three degrees of freedom by three pairs of electromagnetic actuator devices.
According to another aspect of the invention, there is provided a stage that is controllable in the vertical direction by an electromagnetic actuator device, and supported by a supplemental vertical support disposed underneath the stage at the same location as the electromagnetic actuator device. This configuration prevents distortion of the stage due to excessive vertical supplemental force. Preferably, the electromagnetic actuator device comprises three pairs of electromagnetic actuator devices each comprising an actuating portion of variable reluctance and a corresponding target portion for control in three vertical degrees of freedom. Each pair is accompanied by the supplemental vertical support. The stage is suspended from a bar that is supported by the supplemental vertical support using a wire running through a hole formed in the actuating portion and connecting the target portion mounted on the stage and the bar. This configuration allows the target portion to make small horizontal and rotational motions without affecting the position of the bar supported by laterally stiff supplemental vertical support.
These and other feature are more fully described in the detailed examples which follow.